Memory system capable of reducing the reading time

ABSTRACT

A bias circuit includes a charging current reproduce unit, a cell current reproduce unit, a current comparator, and a bit line bias generator. The charging current reproduce unit generates a charging reference voltage according to a charging current flowing through a voltage bias transistor. The cell current reproduce unit generates a cell reference voltage according to a cell current flowing through a common source transistor. The current comparator includes a first current generator for generating a replica charging current according to the charging reference voltage, and a second current generator for generating a replica cell current according to the cell reference voltage. The bit line bias generator generates a bit line bias voltage to control a page buffer for charging a bit line according to a difference between the replica charging current and the replica cell current.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/436,926, filed Jun. 11, 2019, which is a continuation of PCTApplication No. PCT/CN2019/085219 filed on Apr. 30, 2019, both of whichare incorporated herein by reference in their entireties.

BACKGROUND

The present disclosure is related to a memory system, and moreparticularly to a memory system capable of reducing the reading time.

In a memory system, the data stored in the memory cell is usually readby sensing the data voltage on the bit line caused by the memory cell.For example, in a NAND memory read sequence, to read the data stored ina memory cell, the bit line coupled to the memory cell may bepre-charged to a predetermined level first. After the voltage of the bitline has settled, the word line coupled to the memory cell may be raisedto cause the memory cell to generate current according to the datastored in the memory cell. If the memory cell has not been programmed,the memory cell may generate a significant current that pulls down thevoltage of the bit line. Otherwise, if the memory cell has beenprogrammed, the memory cell will not generate any currents or will onlygenerate insignificant current so the voltage of the bit line willremain at the similar level. Therefore, by sensing the voltage of thebit line, the data stored in the memory cell can be read.

However, since the bit line is resistive and capacitive due toinevitable parasitic resistors and capacitors, the settling time of thebit line will contribute to a significant part of the total readingtime. Furthermore, since the resistive and capacitive characteristicsare unpredictable and varied with process, the settling time required bydifferent memory cells are also different. Therefore, the worst casesettling time is always applied to ensure the sensing accuracy. Inaddition, in prior art, the bit line is pre-charged with a master-slavetransistor controlled by a predetermined voltage. In this case, thecharging ability may decrease as the voltage of the bit line approachingto the desired level, which also increases the reading time.

SUMMARY

One embodiment of the present disclosure discloses a memory system. Thememory system includes a plurality of memory cells, a voltage biastransistor, a page buffer, a common source transistor, and a biascircuit.

The first memory cells are coupled to a bit line. The voltage biastransistor has a first terminal for receiving a first system voltage, asecond terminal, and a control terminal for receiving a first biasvoltage.

The page buffer is coupled to the bit line and the second terminal ofthe voltage bias transistor. The page buffer charges the first bit lineto the first system voltage according to a bit line bias voltage duringa pre-charge operation, and forms a sensing path from the first bit lineto a sensing amplifier during a sense operation.

The common source transistor has a first terminal coupled to the firstbit line, a second terminal for receiving a second system voltagesmaller than the first system voltage, and a control terminal forreceiving a control signal.

The bias circuit includes a charging current reproduce unit, a cellcurrent reproduce unit, a current comparator, and a bit line biasgenerator. The charging current reproduce unit is coupled to the voltagebias transistor. The charging current reproduce unit generates acharging reference voltage according to a charging current flowingthrough the voltage bias transistor. The cell current reproduce unit iscoupled to the common source transistor. The cell current reproduce unitgenerates a cell reference voltage according to a cell current flowingthrough the common source transistor.

The current comparator is coupled to the charging current reproduce unitand the cell current reproduce unit. The current comparator includes afirst current generator, and a second current generator. The firstcurrent generator generates a replica charging current according to thecharging reference voltage, and the second current generator generates areplica cell current according to the cell reference voltage.

The bit line bias generator is coupled to the current comparator and thefirst page buffer. The bit line bias generator generates the bit linebias voltage according to a difference between the first replicacharging current and the first replica cell current.

Another embodiment of the present disclosure discloses a bias circuit.The bias circuit includes a charging current reproduce unit, a cellcurrent reproduce unit, a current comparator, and a bit line biasgenerator.

The charging current reproduce unit is coupled to a voltage biastransistor, and generates a charging reference voltage according to acharging current flowing through the voltage bias transistor. The cellcurrent reproduce unit is coupled to a common source transistor, andgenerates a cell reference voltage according to a cell current flowingthrough the common source transistor.

The current comparator is coupled to the charging current reproduce unitand the cell current reproduce unit. The current comparator includes afirst current generator, and a second current generator. The firstcurrent generator generates a replica charging current according to thecharging reference voltage, and the second current generator generates areplica cell current according to the cell reference voltage.

The bit line bias generator is coupled to the current comparator and apage buffer, and generates a bit line bias voltage to control the pagebuffer for charging a bit line according to a difference between thereplica charging current and the replica cell current.

The plurality of first memory cells are coupled to the bit line, thevoltage bias transistor has a first terminal for receiving a firstsystem voltage, a second terminal, and a control terminal for receivinga first bias voltage. The page buffer is coupled to the bit line and thesecond terminal of the voltage bias transistor, and charges the bit lineto the first system voltage according to the bit line bias voltageduring a pre-charge operation. The common source transistor has a firstterminal coupled to the bit line, a second terminal for receiving asecond system voltage smaller than the first system voltage, and acontrol terminal for receiving a control signal.

These and other objectives of the present disclosure will no doubtbecome obvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a memory system according to one embodiment of the presentdisclosure.

FIG. 2 shows a bias circuit according to one embodiment of the presentdisclosure.

DETAILED DESCRIPTION

FIG. 1 shows a memory system 100 according to one embodiment of thepresent disclosure. The memory system 100 includes a plurality of memorycells MC(1,1) to MC(M,N), a voltage bias transistor 110, page buffers1201 to 120N, a common source transistor 120, and a bias circuit 130,where M and N are positive integers.

In FIG. 1, the memory cells MC(1,1) to MC(M,N) are disposed as an array.For example, the memory cells MC(1,1) to MC(M,1) can be coupled to thebit line BL1, and the memory cells MC(N) to MC(M,N) can be coupled tothe bit line BLN. Also, the memory cells MC(1,1) to MC(N) can be coupledto the word line WL1, and the memory cells MC(M,1) to MC(M,N) can becoupled to the word line WLM.

The voltage bias transistor 110 has a first terminal for receiving afirst system voltage VS1, a second terminal, and a control terminal forreceiving a first bias voltage VB1. The first bias voltage VB1 can turnon the voltage bias transistor 110 to charge the bit lines BL1 to BLNthrough the page buffers 1201 to 120N.

The page buffers 1201 to 120N can have the same structure. For example,the page buffer 1201 can be coupled to the bit line BL1 and the secondterminal of the voltage bias transistor 110. The page buffer 1201 cancharge the bit line BL1 to the first system voltage VS1 according to abit line bias voltage VBLB during the pre-charge operation, and can forma sensing path from the bit line BL1 to a sensing amplifier during thesense operation.

In FIG. 1, the page buffer 1201 includes transistors M1 to M5. Thetransistor M1 has a first terminal coupled to the second terminal of thevoltage bias transistor 110, a second terminal, and a control terminalfor receiving a pre-charge control signal SIG_(C1). The transistor M2has a first terminal coupled to the second terminal of the transistorM1, a second terminal, and a control terminal for receiving a clampingcontrol signal SIG_(C2). The transistor M3 has a first terminal coupledto the second terminal of the second transistor M2, a second terminalcoupled to the bit line BL1, and a control terminal for receiving thebit line bias voltage VBLB. The transistor M4 has a first terminalcoupled to the second terminal of the transistor M2, a second terminalcoupled to the sensing amplifier for sensing, and a control terminal forreceiving a sensing control signal SIG_(C3). The transistor M5 has afirst terminal coupled to the second terminal of the transistor M1, asecond terminal coupled to the second terminal of the transistor M4, anda control terminal for receiving a pre-charge select signal SIG_(C4).

During the pre-charge operation, the transistors M1 and M2 will beturned on, and the transistor M3 will also be turned on to charge thebit line BL1. In some embodiments, the memory system 100 can furtherinclude high voltage passing transistors 1501 to 150N, and the pagebuffers 1201 to 120N can be coupled to the bit lines BL1 to BLN throughthe high voltage passing transistors 1501 to 150N respectively. In thiscase, the high voltage passing transistor 1501 will also be turned on bythe pass signal SIG_(HV) during the pre-charge operation of the bit lineBL1.

Also, during the sense operation, the transistors M1, M2, and M3 may beturned off, and the transistor M4 can be turned on so that the voltageof the bit line BL can be sensed by the sense amplifier. The transistorM5 can be used to select the bit line to be pre-charged according to therequirement.

The common source transistor 130 has a first terminal coupled to the bitlines BL1 to BLN, a second terminal for receiving a second systemvoltage VS2 smaller than the first system voltage VS1, and a controlterminal for receiving a control signal SIG_(ACS).

During the pre-charge operation of the bit line BL1, the voltage biastransistor 110 and the common source transistor 130 can be turned on,and the transistors M1, M2, and M3 of the page buffer 1201 can also beturned on. Therefore, the bit line BL1 can be pre-charged. However, inprior art, as the voltage of the bit line BL1 increases, thegate-to-source voltage applied on the transistor M3 will decrease,thereby weakening the charging ability and increasing the required timefor pre-charging. In the memory system 100, to address this issue, thebias circuit 140 can be used to generate and adjust the bit line biasvoltage VBLB for controlling the transistor M3 according to thecondition of the pre-charging operation.

FIG. 2 further shows the bias circuit 140 according to one embodiment ofthe present disclosure. The bias circuit 140 includes a charging currentreproduce unit 142, a cell current reproduce unit 144, a currentcomparator 146, and a bit line bias generator 148.

The charging current reproduce unit 142 is coupled to the voltage biastransistor 110, and can generate a charging reference voltage Vref1according to a charging current Ichg flowing through the voltage biastransistor 110.

The cell current reproduce unit 144 is coupled to the common sourcetransistor 130, and can generate a cell reference voltage Vref2according to a cell current Icell flowing through the common sourcetransistor 130.

The current comparator 146 is coupled to the charging current reproduceunit 142 and the cell current reproduce unit 144. The current comparator146 includes a first current generator 146A, and a second currentgenerator 146B. The first current generator 146A can generate a replicacharging current I_(rchg1) according to the charging reference voltageVref1, and the second current generator 146B can generate a replica cellcurrent I_(rcell1) according to the cell reference voltage Vref2.

The bit line bias generator 148 is coupled to the current comparator 146and the page buffers 1201 to 120N. The bit line bias generator 148 cangenerate the bit line bias voltage VBLB according to a differencebetween the replica charging current I_(rchg1) and the replica cellcurrent I_(rcell1).

In some embodiments, part of the charging current Ichg flowing throughthe voltage bias transistor 110 may flow to the parasitic capacitors onthe bit lines BL1 to BLN in the beginning of the pre-charge operationwhile the rest of the charging current Ichg will flow through the commonsource transistor 130. Later, when the parasitic capacitors are charged,the charging current Ichg will all flow through the common sourcetransistor 130.

That is, in the beginning of the pre-charge operation, the chargingcurrent Ichg would be greater than the cell current Icell, and, thus,the replica charging current I_(rchg1) should be greater than thereplica cell current I_(rcell1). In this case, the difference betweenthe replica charging current I_(rchg1) and the replica cell current Irmawill cause the bit line bias generator 148 to raise the bit line biasvoltage VBLB so the transistor M3 can be fully turned on, therebyincreasing the charging ability.

Later, when the parasitic capacitors are charged completely, the replicacharging current I_(rchg1) will be substantially equal to the replicacell current I_(rcell1). In this case, it may imply that the bit lineBL1 has been charged so the bit line bias generator 148 will keep thebit line bias voltage VBLB, and the sense operation can be performedcorrespondingly.

In some embodiments, the current comparator 146 can further include athird current generator 146C, a fourth current generator 146D, and aninverter 146E for generating a sensing indication signal SIG_(IDCT). Thethird current generator 146C can generate a replica charging currentI_(rchg2) according to the charging reference voltage Vref1, and thefourth current generator 146D can generate a replica cell currentI_(rcell2) according to the cell reference voltage Vref2. The inverter146E has an input terminal coupled to the third current generator 146Cand the fourth current generator 146D, and an output terminal foroutputting the sensing indication signal SIG_(IDCT) according to thedifference between the replica charging current I_(rchg2) and thereplica cell current I_(rcell2). In this case, the sensing indicationsignal SIG_(IDCT) will be flipped when the difference between thereplica charging current I_(rchg2) and the replica cell currentI_(rcell2) becomes zero, and the sense operation can be triggered by theflipped sensing indication signal SIG_(IDCT) accordingly.

Since the bit line bias generator 148 can adjust the bit line biasvoltage VBLB according to the charging status of the bit lines BL1 toBLN instantly, the charging ability can be maintained to be strongduring the pre-charge operation. Also, since the charging status of thebit lines BL1 to BLN can be detected by the difference between thereplica charging current I_(rchg1) and the replica cell currentI_(rcell1), the pre-charge operation can be terminated and the senseoperation can be triggered once the bit lines BL1 to BLN arepre-charged. That is, the pre-charge time can be optimized, and thepre-charge operation can be controlled without being affected by theprocess variation.

In FIG. 2, the charging current reproduce unit 142 includes transistorsM6 and M7, and an operational amplifier OP1. The transistor M6 has afirst terminal for receiving the first system voltage VS1, a secondterminal, and a control terminal coupled to the control terminal of thevoltage bias transistor 110. The operational amplifier OP1 has apositive input terminal coupled to the second terminal of the transistorM6, a negative input terminal coupled to the second terminal of thevoltage bias transistor 110, and an output terminal for outputting thecharging reference voltage Vref1. The transistor M7 has a first terminalcoupled to the second terminal of the transistor M6, a second terminalfor receiving the second system voltage VS2, and a control terminalcoupled to the output terminal of the operational amplifier OP1.

In this case, the operational amplifier OP1 can ensure the transistor M6to be biased under the same condition as the voltage bias transistor110. Therefore, the charging current reproduce unit 142 is able togenerate a reproduce current according to the charging current Ichgflowing through the voltage bias transistor 110.

Similarly, the cell current reproduce unit 144 includes transistors M8and M9, and an operational amplifier OP2. The transistor M8 has a firstterminal for receiving the first system voltage VS1, a second terminal,and a control terminal. The operational amplifier OP2 has a positiveinput terminal coupled to the second terminal of the transistor M8, anegative input terminal coupled to the bit lines BL1 to BLN, and anoutput terminal coupled to the control terminal of the transistor M8 foroutputting the cell reference voltage Vref2. The transistor M9 has afirst terminal coupled to the second terminal of the transistor M8, asecond terminal for receiving the second system voltage VS2, and acontrol terminal coupled to the control terminal of the common sourcetransistor 130.

In this case, the operational amplifier OP2 can ensure the transistor M9to be biased under the same condition as the common source transistor130. Therefore, the cell current reproduce unit 144 is able to generatea reproduce current according to the cell current Icell flowing throughthe common source transistor 130.

In FIG. 2, the first current generator 146A includes a transistor M10having a first terminal, a second terminal for receiving the secondsystem voltage VS2, and a control terminal for receiving the chargingreference voltage Vref1. Also, the second current generator 146Bincludes a transistor M11 having a first terminal for receiving thefirst system voltage VS1, a second terminal coupled to the firstterminal of the transistor M10, and a control terminal for receiving thecell reference voltage Vref2.

In addition, in FIG. 2, the transistors M7 and M10 are N-typetransistors while the transistors M8 and M11 are P-type transistors. Inthis case, the transistor M10 will be biased under the same condition asthe transistor M7 with the charging reference voltage Vref1, so thetransistor M10 can generate the replica charging current I_(rchg1) bymirroring the current flowing through the transistor M7. Similarly, thetransistor M11 will be biased under the same condition as the transistorM8 with the cell reference voltage Vref2, so the transistor M11 cangenerate the replica cell current I_(rcell1) by mirroring the currentflowing through the transistor M8.

In FIG. 2, the bit line bias generator 148 includes an operationalamplifier OP3, a transistor M12, and a resistor R1. The operationamplifier OP3 has a positive input terminal for receiving a second biasvoltage VB2, a negative input terminal coupled to the first terminal ofthe transistor M10, and an output terminal for outputting the bit linebias voltage VBLB. The transistor M12 has a first terminal coupled tothe output terminal of the operation amplifier OP3, a second terminalcoupled to the negative input terminal of the operation amplifier OP3,and a control terminal coupled to the first terminal of the transistorM12. The resistor R1 has a first terminal coupled to the second terminalof the transistor M12, and a second terminal for receiving the secondsystem voltage VS2.

In this case, when the replica charging current I_(rchg1) is greaterthan the replica cell current I_(rcell1), a differential currentI_(diff) will be fed to the bit line bias generator 148, thereby pullingdown the voltage of the negative input terminal of the operationalamplifier OP3 and raising the bit line bias voltage VBLB.

In some embodiments, the ratio of the size of the transistors M7 and M10can be selected according to the system requirement to adjust thereplica charging current I_(rchg1). However, the ratio of the size ofthe transistors M8 and M11 should be the same as the ratio of the sizeof the transistors M7 and M10.

Similarly, the ratio of the size of the transistor M6 and the voltagebias transistor 110 can be selected according to the system requirement,and the ratio of the size of the transistor M6 and the voltage biastransistor 110 should be the same as the ratio of the size of thetransistor M9 and the common source transistor 130.

Furthermore, in FIG. 2, the charging current reproduce unit 142 and thecell current reproduce unit 144 can use the operational amplifiers OP1and OP2 to fix the bias conditions firmly; however, in some otherembodiments, the charging current reproduce unit 142 and the cellcurrent reproduce unit 144 may be implemented with other structures,such as the commonly used current mirrors.

Also, in FIG. 1, the bit lines BL1 to BLN can be pre-charged at the sametime, however, in some other embodiments, the bit lines BL1 to BLN mayalso be pre-charged independently with the page buffers 1201 to 120Naccording to the system requirement.

In summary, the memory system and the bias circuit provided by theembodiments of the present disclosure can adjust the bit line biasvoltage according to the charging status of the bit lines instantly, sothe charging ability can be maintained to be strong during thepre-charge operation. Also, since the charging status of the bit linescan be detected by the difference between the replica charging currentand the replica cell current, the pre-charge time can be optimized, andthe pre-charge operation can be controlled without being affected by theprocess variation.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the disclosure. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. A memory system comprising: a plurality of firstmemory cells coupled to a first bit line; a voltage bias transistorhaving a first terminal configured to receive a first system voltage, asecond terminal, and a control terminal; a first page buffer coupled tothe first bit line and the second terminal of the voltage biastransistor; a common source transistor having a first terminal coupledto the first bit line, a second terminal configured to receive a secondsystem voltage, and a control terminal; and a bias circuit comprising: acharging current reproduce unit coupled to the voltage bias transistor,and configured to generate a charging reference voltage according to acharging current flowing through the voltage bias transistor; a cellcurrent reproduce unit coupled to the common source transistor, andconfigured to generate a cell reference voltage according to a cellcurrent flowing through the common source transistor; a currentcomparator coupled to the charging current reproduce unit and the cellcurrent reproduce unit, and comprising: a first current generatorconfigured to generate a first replica charging current according to thecharging reference voltage; and a second current generator configured togenerate a first replica cell current according to the cell referencevoltage; and a bit line bias generator coupled to the current comparatorand the first page buffer, and configured to generate the bit line biasvoltage according to a difference between the first replica chargingcurrent and the first replica cell current.